Thermal microswitch



p -5, 0 Y P. J. CADE, 2,520,906

THERMAL IIICROSWITCH Filed June 22, 1946 Tnvenlnr PHlbL/P J. CADE-li-urngy' Patented Sept. 5, 1950 UNITED STATES PATENT OFFICE THERMALIWICROSWITCH Application June 22, 1946, Serial No. 678,549

3 Claims. 1

The present invention relates in general to switch actuators and moreparticularly to thermally operated actuators for use with switches ofthe snap action type.

The principal object of this invention is to provide an improved snapaction switch actuator.

Another object is to provide a snap action switch actuator that isthermally controlled.

Another object is to provide such an improved snap action switchactuator which is unaffected by changes in ambient temperature.

Another object is to provide an actuator of the type above indicatedwhich is so designed that a standard resistor unit of the highresistance type may be easily attached to it and thereby provide aheater means to operate the actuator.

Another object is to provide an improved actuator of the type aboveindicated which is positive in action and requires only a small amountof power to effect operation.

Another object is to provide an actuator which has an inherent timedelay. I

Other objects and novel features of the 'invention will be apparent asthe description thereof progresses.

In the drawings illustrating the preferred embodiment of the invention:

Fig. l is a side elevation of an actuator device, constructed inaccordance with the invention, mounted on the housing of a snap switchof a conventional type;

Fig. 2 is a top elevation of Fig. 1;

Fig. 3 is a diagram of a simple circuit in which the actuator may beused, and

Fig. 4 is a modified form of the actuator shown in Fig. 1.

Referring more in detail to the drawings, l indicates a conventionalsnap action switch, on the outside casing of which the present device ismounted. If preferred, the casing may be removed and a special mountingmay be provided for both switch and actuator.

For the purpose of the present application, and to simplify thedisclosure it is to be understood that a very slight pressure on thebutton ll projecting through the side of the housing of the switch l0causes the switch contact member I2 to engage with a fixed contactmember I3 (see Fig. 3), thereby effecting a closed circuit, as will bemore fully described further along in the description.

Mounted on the housing of the switch I0 is a post id to which isfastened by means of the screw IS a strip of bimetal l6. Secured to thebimetal strip IB is an electrical and heat insulating strip ll. Anysuitable means, such as rivets I8, may be used to connect these pieces.In a like manner, similar rivets l9, are provided to fasten the oppositeend of the insulator strip II to another bimetal member 20. The bimetalmember 20 is provided with an open loop 2 I. The free end of the bimetalstrip 20 extends rearwardly below the upper portion terminating in aninetydegree bend 2 la, the end of which, when properly proportioned andadjusted, very lightly contacts the button II. The bimetal strips l6 and20 are of any conventional type and are so proportioned and of suchcharacteristics that when both are subjected to the same ambienttemperature the bending of one member compensates for the bending of theother and the position of end 2Ia is not changed.

Fig. 4 discloses a modified form of actuator which is constructed of asingle bimetallic strip and so shaped as to be self-compensating. Inthis embodiment the upper straight portion 25 is approximately equal inlength to the combined lengths of lower straight portion 26, and curvedportion 2|. If it is assumed, for example, that the metal of strip 21has a greater coeflicient of expansion thanstrip 28, it can be seenthat, upon a rise in ambient temperature, the bending of portion 25tends to move leg 2lb upward, and the bending of curved section 2| andstraight section 26 both tend to move point 2lb downward. If the switchis proportioned as indicated above these two motions will exactlycompensate each other so that no pressure is exerted on button II. Ifthe loop 2| alone is heated, however, the loop will expand and exertpressure on button II. In Figs. 1 and 4, a heating resistor 22 which maybe of the composition type commonly used in electronic circuits, andpreferably has a high resistance value, is held within the bimetallicloop. The bimetals are arranged in both cases so that the loop willexpand when heated. It is apparent that when the resistor unit 22 isheated by an electric current the expansion of the loop 2| will depressthe end piece 2ia or 2lb against the snap switch button ll therebycausing the contacts l2 and I3 to become engaged. In Fig. 1, theinsulating strip ll prevents conduction of heat away from the loop,thereby ensuring maximum sensitivity of the actuator, and also isolatesstrip 2| from strip l6 so that strip l6 may be attached directly to onecontact member of the switch if desired.

Fig. 3 illustrates a simple circuit in which the present invention maybe used. A and B represent the terminals of an alternating currentsupply source, to which are connected a potential apportioning network Nwith impedances RI and R2, and an electronic discharge device, forexample a gas-filled cold cathode tube T, with anode a, cathode k andstarter anode c. Impedance Rl may be bridged by variable-impedancedetecting device, for example two electrodes or probes pl, p2 associatedwith a tank t for a liquid 1. The starter anode c is connected to apoint between impedance RI and R2.

The heating resistor 22 of the actuator is connected in the platecircuit of tube T. The fixed contact I! of the snap switch It isconnected to terminal A through an alarm or control device D and movablecontact i2 is connected to the terminal B. The potential apportioningnetwork is so arranged, that, when the probes pi and p2 are above theliquid level and constitute a high impedance, the potential of c is lowenough to prevent the tube from conducting. The snap switch l and thesuperimposed actuator are so arranged that, with the tube Tnon-conductive, and no current flowing through heating resistor 22 thebutton contact end 2la or 2lb of the bimetal member 20 or 23 permits thecontacts to assume a normally open position as shown in the drawing. Nocurrent then flows in 22 and contact l2 remains in its normally openposition. If the water I in the tank t rises until probes pl and p2 areconductively connected, the potential drop across impedance RI will bedecreased, the potential of 0 will rise and tube T becomes conductive.The output current flowing through the resistor 22 in the open loop ofthe bimetal strip 20 heats the resistor. The bimetal loop 2| expandscausing the contact end 2la and 2lb to press against the button ll ofthe switch Ill. Contact member 13' then snaps to the right againstcontact member l2 and closes the circuit through signalling device D. 7

It can readily be seen that ambient temperature compensation of theswitch actuator is essential to the satisfactory operation of anarrangement of this type. Because of the low power generally availablein a tube output circuit it is necessary to use a thermal actuator ofhigh sensitivity and changes in ambient temperature, if not compensatedfor, might be sufiicient to operate the switch and causes falseoperation of the control or alarm. With respect to Fig. 1 suchcompensation may be had by making the metal of the lower part of stripl6 (that which is nearer the housing of switch ID) of a highercoefiicient of expansion than the metal of the upper part, so that stripl6 behaves similarly to the straight portion 25 in Fig. 4.

Since certain changes may be made in the above-described article anddifferent embodiments of the invention could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description or shown in the accompanying drawing shall beinterpreted as illustrative only and not in a limiting sense.

What is claimed is:

1. In a switch having a stationary and a movable contact member, athermal actuator for said movable member comprising: a bimetallic stripin the form of a loop with its 'ends bent into legs of different lengthsprojecting substantially perpendicularly to each other, the longer ofsaid legs being fixed at its outer end to said switch and the shorter ofsaid legs being arranged to engage said movable member, said loop andsaid longer leg being so proportioned as to maintain the position ofsaid shorter leg with respect to said switch substantially unchangedwhen the entire strip is heated to uniform temperature; and a heatingcoil mounted in intimate heat transfer relationship with said loop butspaced from said legs.

2. A thermostatic switch comprising: a base: a fixed contact attached tosaid base; a movable contact; a bimetallic strip in the form of a loopwith its ends bent into legs of different lengths projectingsubstantially perpendicularly to each other, the longer of said legsbeing fixed at its outer end to said base and the shorter of said legsbeing arranged to engage said movable contact, said loop and said longerleg being so proportioned as to maintain the position of said shorterleg with respect to said base substantially unchanged when the entirestrip is heated to uniform temperature; and a heating coil mounted inintimate heat transfer relationship with said loop but spaced from saidlegs.

3. A thermostatic switch comprising: a base; a fixed contact attached tosaid base; a snap action element carrying a movable contact; abimetallic strip in the form of a loop with its ends bent into legs ofdifferent lengths projecting substantially perpendicularly to eachother, the longer of said legs being fixed at its outer end to said baseand the shorter of said legs being arranged to engage said movablecontact, said loop and said longer leg being so proportioned as tomaintain the position of said shorter leg with respect to said basesubstantially unchanged when the entire strip is heated to uniformtemperature; and a heating coil mounted in intimate heat transferrelationship with said loop but spaced from said legs.

PHILLIP J. CADE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,270,356 Wurster June 25, 19181,328,478 Baker Jan. 20, 1920 1,373,128 Freese Mar. 29, 1921 1,780,302Koch et al. Nov. 4, 1930 1,828,321 Hesse Oct. 20, 1931 1,886,354O'Keeffe Nov. 1, 1932 1,886,355 O'Keeffe Nov. 1, 1932 2,087,024 DezotellJuly 13, 1937 2,335,212 Landon Nov. 23, 1943 2,344,946 Landon Mar. 28,1944 FOREIGN PATENTS Number Country Date 440,413 Great Britain Dec. 23,1935

